I don't know about WAY off, the prediction is as good as the data you give it.
FWIW, I have never had my car on a dyno but based on knowing the weight and the 1/4 mile trap speed I can estimate that I have ~370 HP. The Camquest software predicts that I have ~390HP. My 360 is pretty close to the Mopar Performance 360/380HP crate motor which always seems to dyno at more than the advertised 380HP.